: The long-range goal of this research is to determine the cellular and developmental impact of an unusually structured inhibitor of apoptosis which has recently been discovered in Drosophila. Apoptosis and its precisely controlled inhibition are fundamental to the proper formation and development of the dipteran embryo, larva and adult. This new insect apoptosis inhibitor, deterin, occurs in the embryo, larva and reproductive adult female. It has been shown to block apoptosis induced by reaper (an insect-specific apoptosis activator) in both transgenic flies and cultured cells. The impact of deterin on insectan developmental processes and apoptotic regulators can only be determined in an insect system. This will be done by addressing three specific aims. 1.) The precise temporal and tissue specific expression of the deterin transcript and its encoded protein will be determined in developmental studies. 2.) Hypotheses about the developmental functions and antiapoptotic actions of drosophilan and mosquito deterin during embryonic, metamorphic and reproductive development and in normal and apoptotic cells will be tested. 3.) The molecular mechanism of action of the deterin protein, including its target sites will be assessed.